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Alkali metals when the amount of air is not

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Unformatted text preview: t can be judged based on radii but for ∆Hf we have to know other values as well CHMB31H3 The Chemistry of Group 1 12 Oxides (MxOy) • A variety of oxides known particularly for heavier alkali metals • When the amount of air is not controlled: 4 Li + O2 → 2 Li2O (‘normal’ oxide) 2 Na + O2 → Na2O2 (peroxide, O22-) K + O2 → KO2 (superoxide, O2-) • Rb and Cs give mixtures predominantly containing superoxides • Normal oxides (M2O) of Na, K, Rb and Cs can be obtained from O2 and M under controlled conditions or thermal decomposition of peroxides and superoxides (with liberation of O2) • Thermal decomposition of Group 1 nitrates can give normal oxides 800℃ (commonly used for synthesis of Na2O) : 4 NaNO3 CHMB31H3 2Na2O + 5O2 + 2N2 The Chemistry of Group 1 13 Oxides (cont.) • The stability of peroxides increases from Li2O2 to Cs2O2 → This observation shows another important effect: large anions can be stabilized by large cations and vice versa. • Partial oxidation of Rb and Cs gives several suboxides with interesting structures: • Note that the oxidation number of Cs and Rb in these compounds is well bellow +1 expected fro the Group 1 metals (i.e. ON for Cs in Cs11O3 is +6/11or ≈ + ½). CHMB31H3 The Chemistry of Group 1 14 Oxides (reaction with H2O) and hydroxides • All oxides are basic oxides and react with acids and acid oxides to produce salts → Remember that the oxides of typical metals are basic! • Reaction of all oxide types with water produces hydroxides (MOH) but...
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